Reverse-osmosis (RO) is a liquid filtration technique that is suitable for a wide range of applications. For example, RO filtration is used in water purification processes to remove salts and other effluent from seawater or brackish water solutions. In a typical RO purification process, an RO separator module having an RO membrane is placed in a pressure vessel in which a supply of feed liquid (e.g., seawater) is pressurized against one side of the membrane. The RO membrane rejects the solutes in the feed liquid (e.g., salt and other effluent) to produce a concentrate liquid on the one side, and enables transport of a solute-free permeate liquid (e.g., salt-and-effluent-free water) across the membrane to the other side. The permeate liquid and concentrate liquid are each collected from the RO separator module through separate flow paths.
In some applications, such as drinking water purification, the permeate liquid collected from the RO separator module may be sufficiently free of salt or other effluent after a single pass through the RO separation medium. For other applications, such as semiconductor or electronics manufacturing, the permeate liquid must be considerably cleaner than what is typically capable of being achieved through a single pass. Traditionally, these more demanding applications use multiple RO separator modules connected downstream in series, such that the permeate liquid exiting an upstream RO module is then used as the feed liquid entering a separate downstream RO module for further purification. These daisy-chained configurations can take up considerable manufacturing floor space and can require extensive plumbing, which increases the costs associated with purifying the processing liquid.